Abstract

Professor R. M. Shackleton remarked that the authors' presentation of the comparison between the Southern Uplands structures and those in an accreting prism next to a trench was slightly misleading; the structures appeared similar only because the vertical scale, and therefore the dip of the slices, was greatly exaggerated in the illustration of the present-day prism and subduction zone. The speaker accepted that the Southern Uplands structures probably are analogous to those of a presently accreting prism, but if so one must suppose that the present steep NW facing attitude of the sedimentary slices in the Southern Uplands must be the result of a drastic northwestward rotation of thrust slices that were initially gently inclined. Since the angle between the steep separating faults and the steeply dipping beds is small, the slices must extend down a long way in comparison to their thickness. Restored to their initial positions, before they were stacked by thrusting and rotated to verticality, they must have extended many hundreds of km southeastwards across the ocean floor.

Dr J. K. Leggett replies: There is no difficulty in explaining the rotation, through prolonged subduction and accretion, of initially low angle decollement planes in mature accretionary forearc complexes. Hence there is no problem in explaining the vecticality of the structural units in the Northern Belt of the Southern Uplands. However, Professor Shackleton's point concerning the strata in the southern portion of the Southern Upland is well taken: in most cases they have essentially the same attitude as their